Texture development in S200-D, -E and P31664 beryllium blocks from neutrondiffraction spectra

The mechanical behavior of beryllium is important in a range of Department of Energy (DOE) weapons applications. Its unique high elastic modulus, modu lus-to-density ratio and strength-to-density ratio make beryllium an ideal material for aerospace vehicles and weapons components (1-3). Because of it s unique properties and the range of important applications, beryllium has already been extensively investigated. Strain rate sensitivity, work harden ing, temperature-dependent viscosity, anisotropic fracture, grain size effe cts and oxide content are a few physical and chemical factors that have bee n measured and collated to create a database allowing prediction of its con stitutive properties (4-8). Finite element codes have been used to simulate stress during crack propagation in metals that may be applied to beryllium (9). However, successful utilization of beryllium depends on an accurate d escription of its microstructure as a function of fabrication and forming h istory. Texture, preferred crystallographic orientation, is an important mi crostructural parameter that we use in this paper to characterize structure as a function of manufacturing process for three archived beryllium struct ural grades. Los Alamos National Laboratory (LANL) maintains a stock of various vintages of structural (nuclear grade) beryllium which has been made by a variety o f fabrication processes (3). Texture in beryllium has been observed qualita tively in previous studies, however it has never been investigated systemat ically or quantitatively (10,8). In this study we determine quantitatively the texture as a function of processing history in specimens cut from three vacuum hot pressed (VHP) block beryllium grades S200-D, -E, and P31664 whi ch have been used in various components over the years. Hot pressing of metal powders has been known to produce preferential alignm ent of crystals during formation (11). Moreover, it has been shown that the standard production method of beryllium powder creates anisomorphic grain shape and preferred alignment of crystals (3). The vintages in this study w ere hot-pressed from processed beryllium powder, and therefore texture was anticipated. Texture is directly linked to anisotropic physical properties of (12). For predicting material anisotropic fracture behavior and flow in beryllium, a knowledge of texture is critical. It has long been recognized that single c rystals of beryllium exhibit highly anisotropic behavior (for a through rev iew see (13)). The deformation of beryllium at ambient temperature occurs b y motion of dislocations with burgers vectors of (1 1 (2) over bar 0), the direction of closest atom packing. Such dislocations can move on the (0001) plane, which is the closest packed plane. They can also move on (10 (1) ov er bar 0) prism planes, often at much higher resolved shear stress. The ope ration of (1 1 (2) over bar 0) (10 (1) over bar 0), temperatures. Character izing beryllium texture quantitatively performance prediction of components made from it. temperature. of one or the ether systems, (1 1 (2) over bar 0) (0001) or 1 will depend on which system is most favorably oriented to th e direction of applied stress (14). The brittleness of polycrystalline bery llium results from two circumstances. First, the number of independent slip systems is not sufficient to satisfy the Von Mises requirement of 5 indepe ndent slip systems in order re, maintain grain-to-grain compatibility. Coin cidentally, at ambient temperature there are no mobile dislocations with a c-axis component. Secondly, cleavage fracture is I 1 also common in berylli um because the stress for cleavage on bbasal (0001) planes is quite low at ambient temperatures. Characterizing beryllium texture quantitatively is th us crucial fur mechanical analysis and Processing of vacuum cast beryllium into powder has been historically carri ed out by Braun-type attrition methods when a machined beryllium swarf is g round between two opposed beryllium disks to a powder that is then mechanic ally sieved inside a vacuum. Particles less than 44 micron diameter are the n used for consolidation. This method was used for both S200-D and -E vinta ges. This method produces flattened powder particles because basal cleavage is favored. In the 1970's the method for processing of vacuum cast berylli um into powder was changed in order to increase the mechanical isotropy of the material (by producing more isomorphic grains, thus decreasing the diff erence between longitudinal and transverse properties). New methods such as impact grinding and ball milling were introduced to beryllium powder proce ssing. During impact grinding a beryllium swarf is Do-ground, usually by pi n milling, and pre-ground particles are accelerated through a gas stream, i mpacting on a beryllium target. Particles less than 44 microns in diameter are collected by a cyclone separator for consolidation. While impact grindi ng and Braun-type attrition produced similar grain-size beryllium powders, they resulted in very different grain morphologies. The latter method produ ced flat pancake-like grains and the former produced round grains (3). The three vintages in this study were all vacuum hot pressed (VHP). In this procedure beryllium powder is compacted under uniaxial pressure at high In this paper we present a quantitative texture analysis of 14 specime ns cut from either S200-D -E or P31664 vintage billets from the LANL archiv e. The billets were sectioned for time-of-night (TOF) neutron diffraction m easurements to allow comparison of microstructural differences between vint ages. Neutron diffraction is ideal for investigating structural propel ties in beryllium because the neutron absorption of beryllium is negligible and bulk (2 cm(3)) specimens can therefore be investigated. Moreover, the poly chromatic TOF radiation in this study provides significant statistical adva ntages for texture measurements compared to the traditional method of monoc hromatic x-ray diffraction. Even order harmonic coefficients are extracted from a combination of 50 - 60 TOF neutron diffraction spectra containing a total of 40,000-60,000 data points via a Rietveld refinement method (15). S elected pole figures are subsequently calculated from these harmonic coeffi cients.